This project will investigate a role of EphB receptor signaling in the maintenance of mature dendritic spines and synapses. Dendritic spines are small protrusions on the surface of the dendrite that receive the majority of excitatory synapses in the brain and play a critical role in learning and memory. Defects in dendritic spine morphology have been found in the brains of patients with neurodevelopmental disorders associated with mental retardation and autism, including Rett Syndrome, Down Syndrome, Angleman's Syndrome and Fragile X Syndrome, that exhibit immature dendritic spine profiles. Moreover, dendritic spine loss is a hallmark of several neurodegenerative diseases associated with memory loss. Our previous studies demonstrate that EphB receptors play an important role in dendritic spine formation. We have shown that EphB receptor forward signaling induces formation of mature dendritic spines; whereas hippocampal neurons deficient for three EphB receptors fail to form dendritic spines in vitro and develop abnormal immature spines in vivo; (Ethell et al., 2001; Henkemeyer et al., 2003; Moeller et al., 2006). However, the role of trans-synaptic ephrinB-EphB receptor signaling and EphB2-mediated regulation of GTPase activity in maintaining mature dendritic spines remains unclear. We hypothesize that long- lasting EphB receptor signaling in synapses is also important for the stabilization of mature dendritic spines and can be regulated by matrix metalloproteinases (MMPs). Our proposed mechanism for EphB- mediated dendritic spine stabilization relies on the ability of EphB receptors (1) to activate downstream mechanisms that stabilize existing dendritic spines through the LIMK1-pCofilin pathway and (2) to inhibit integrin-mediated signaling that encourages dendritic spine remodeling. In contrast, MMPs convert the adhesive/stabilizing effects of EphB receptors in dendritic spines into repulsion and enhance integrin- mediated signaling resulting in dendritic spine remodeling. In order to test this hypothesis, we will employ established and novel techniques to complete three specific aims. Specific aim 1 will establish the role of EphB receptors in the maintenance and LIMK1-pCofilin-dependent stabilization of mature dendritic spines and synapses. Specific aim 2 will determine whether EphB receptors act as competitive inhibitors of integrin signaling in dendritic spines. Specific aim 3 will evaluate a role for the cleavage of EphB receptors by MMPs in dendritic spine/synapse remodeling in hippocampal neurons. EphB receptors are promising therapeutic targets for several neurological disorders. While complete ablation and blockade of EphB receptor functions have drastic effects on dendritic spines and synapses, moderate means to regulate EphB receptor signaling may allow for development of effective therapeutics to treat these disorders.